These cooperative systems comprise a so-called secondary surveillance radar and airborne transponders aboard the aircraft. The secondary radar cooperates according to a determined protocol with airborne transponders (also called responders) aboard the aircraft. The secondary radar comprises an interrogator that emits interrogation pulses modulated in amplitude and in phase at the frequency of 1030 MHz so as to establish a communication with the transponders present in the emission lobe of its antenna. The transponders present in the antenna lobe respond with trains of pulses modulated in amplitude at the frequency of 1090 MHz. These pulses are received and processed by a receiver of the secondary radar.
Secondary radars are used both in civil and military applications, in the guise of surveillance radar (known as “Secondary Surveillance Radar” or SSR) or anti-collision radar (airborne radar). The International Civil Aviation Convention, also known as the ICAO (International Civil Aviation Organization), defines a communication protocol for secondary radars in its annex 10 (Aeronautical Telecommunications), Volume IV (Surveillance radar and anti-collision systems). The ICAO standard defines several modes of interrogation, such as modes A, C and S. Mode S is distinguished from modes A and C in that it allows selective interrogation of aircraft by employing an identification number specific to each aircraft. However, the mode S interrogations and responses are long as compared with the mode A or C interrogations and responses.
A mode S response is made up of a train of pulses comprising a preamble and a message. The preamble comprises four pulses of duration 0.5 μs each. The first two and the last two pulses are separated by 0.5 μs. The first and the third pulse are separated by 3.5 μs.
The responses to the selective interrogations (mode S) are of such a nature as to induce numerous false detections of secondary responses (mode A or C). The false detections are also called detections of ghost responses, that is to say of a signal having the form of a response, but not corresponding to any real response. Thus, the message of a mode S response can contain sequences of pulses having the form of a secondary response (mode A or C). Moreover, when the signals are deformed by multipaths, the mode S responses may induce numerous false detections of secondary responses. The false detections increase the processing load of the secondary radar. They may even create an overload leading to nondetection of correct responses.
Upon the appearance of the selective interrogations in the ICAO standard, the need to filter the mode S responses had made itself felt, to make it possible to detect the secondary responses with a minimum of false detections. Patent Application FR 2 692 995 filed on Jun. 30, 1992 describes a method of filtering mode S responses making it possible to preserve the secondary responses received during the time of the filtered mode S responses. This method is based on the selective elimination of the pulses belonging to a mode S response.
Be that as it may, this method, as well as the other methods of filtering mode S responses, carry out a prior detection of the mode S responses. The prior detection is performed in the presence of the four pulses of the preamble.
The filtering methods using the pulses of the preamble are no longer effective when pulses of the preamble are impaired or absent. Still, such a situation is common in the case of temporal overlap of responses at the receiver level, also known as garbling. Specifically, it may happen that the preamble of a mode S response is mixed with another secondary or mode S response, rendering this preamble unidentifiable.